Manual

DS2784: 1-Cell Stand-Alone Fuel Gauge IC with Li+ Protector and SHA-1 Authentication
CAPACITY ESTIMATION ALGORITHM
Remaining capacity estimation uses real-time measured values, stored parameters describing the cell
characteristics, and application operating limits. Figure 2 describes the algorithm inputs and outputs.
Figure 2. Top-Level Algorithm Diagram
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FuelPack is a trademark of Dallas Semiconductor, a wholly owned subsidiary of Maxim Integrated Products, Inc.
Capacity Look-up
A
vailable Capacity Calculation
A
CR Housekeeping
A
ge Estimato
r
Learn Function
Cell
Model
Parameters
(EEPROM)
FULL(T) (R)
A
ctive Empty (T) (R)
Standby Empty (T) (R)
Remaining Active Absolute
Capacity (RAAC) mAh (R)
Sense Resistor’
(RSNSP) (1byte EE)
Voltage (R)
Temperature (R)
Current (R)
A
ccumulated
Current (ACR) (R/W)
User Memory (EEPROM)
16 bytes
A
ging Cap (AC)
(2 bytes EE)
Charge Voltage
(VCHG) (1 byte EE)
Remaining Standby Absolute
Capacity (RSAC) mAh (R)
Remaining Active Relative
Capacity (RARC) % (R)
Remaining Standby Relative
Capacity (RSRC) % (R)
A
ge Scalar (AS)
(1 bytes EE)
Min Chg Current
(IMIN) (1 byte EE)
Empty Voltage
(VAE) (1 byte EE)
Empty Current (IAE)
(1 byte EE)
A
verage Current (R)
MODELING CELL CHARACTERISTICS
To achieve reasonable accuracy in estimating remaining capacity, the cell performance characteristics
overtemperature, load current, and charge-termination point must be considered. Since the behavior of Li+ cells is
nonlinear, these characteristics must be included in the capacity estimation to achieve an acceptable level of
accuracy in the capacity estimation. The FuelPack™ method used in the DS2784 is described in general in
Application Note 131: Lithium-Ion Cell Fuel Gauging with Dallas Semiconductor Battery Monitor ICs. To facilitate
efficient implementation in hardware, a modified version of the method outlined in AN131 is used to store cell
characteristics in the DS2784. Full and empty points are retrieved in a lookup process which retraces a piece-wise
linear model consisting of three model curves named full, active empty, and standby empty. Each model curve is
constructed with 5-line segments, numbered 1 through 5. Above 40°C, the segment 5 model curves extend
infinitely with zero slope, approximating the nearly flat change in capacity of Li+ cells at temperatures above 40°C.
Segment 4 of each model curves originates at +40C on its upper end and extends downward in temperature to the
junction with segment 3. Segment 3 joins with segment 2, which in turn joins with segment 1. Segment 1 of each
model curve extends from the junction with segment 2 to infinitely colder temperatures. The three junctions or
breakpoints that join the segments (labeled TBP12, TBP23, and TBP34 in Figure 3) are programmable in 1°C
increments from
-128°C to +40°C. The slope or derivative for segments 1, 2, 3, and 4 are also programmable over a range of 0 to
15,555ppm, in steps of 61ppm.